Moisture Ingress into O-rings

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1 Moisture Ingress into O-rings David Leslie, Abhijit Dasgupta CALCE, University of Maryland College Park, MD Hans de Vries, Hans Mollen Phillips Research Labs, Eindhoven, NL ASTR 2012 Oct , Toronto, Ontario, Canada Moisture Modeling 1

2 Background and Objectives Background O-rings have been used since the early 1900s as a sealant It is necessary to understand their moisture permeation characteristics Objectives Characterize diffusivity coefficients of O-ring materials Use FEA modeling to obtain correction factors: 1-D Model (JEDEC JESD22-A120A) 3-D Diffusion in Toroidal Geometries Moisture Modeling 2

3 Procedure Scaffold HAST Saturation Phase (105C/100%RH): HAST chamber used O-Rings hung using metals hangers Balance Oven Desorption Phase (Constant T): Temperature controlled oven used Samples weighed periodically outside the oven under ambient conditions Moisture Modeling 3

4 Results Desorption (25 C) 1.00 Saturation (%) Silicone EPDM The beginning linear portion is used to extract the diffusivity. The start and end weights are used to extract solubility information Time ( x 1000 s) * The desorption process was done under ambient RH conditions. Moisture Modeling 4

5 Results Saturation (%) Desorption (70 C) EPDM Silicone Time ( x 1000 s) At 25 C, silicone loses 50% moisture about 1.5x faster than EPDM 50% moisture loss in silicone at 70 o C is 9 to 10 times faster than at 25 C In comparison, moisture loss in EPDM is only 2 to 3 times faster * The RH was consistently below 5% in the desorption oven. Moisture Modeling 5

6 Fick s Second Law (1D): φ/ t =D 2 φ/ x 2 Diffusivity Coefficient (1D):* Analysis: 1D Diffusion D is the diffusivity coefficient (m 2 /s) φ is the concentration (g/m 3 ) t is time (s) x is position (m) M ω (% saturation) is the equilibrium moisture concentration Corresponds to the initial value on the diffusion curve M 1 is the moisture content after time t 1 (s) M 2 is the moisture content after time t 2 (s) h is the thickness (m) * Wetness: W(t)= M(t)/ M ω W(0)= M(0)/ M ω =1 (W(t) W(0)) 2 = ( M 2 M 1 / M ω ) 2 Moisture Modeling 6

7 Analysis 1D Diffusivity Calculation (25 C) (W(t) - W(0))^ Silicone y = 6.57E-06x y = 3.68E-06x EPDM Time ( x 1000 s) Moisture Modeling 7

8 Analysis 1D Diffusivity Calculation (70 C) Silicone y = 6.15E-05x (W(t) - W(0))^ EPDM y = 1.21E-05x Time ( x 1000 s) Moisture Modeling 8

9 Results 1D Approximation Diffusivity (m 2 /s) Materials 25 C 70 C Silicone (1.6 ± 0.1) x (1.5 ± 0.1) x EPDM (1.2 ± 0.3) x (3.8 ± 0.2) x Uncertainty ranges are estimated from uncertainties in: weight measurement time of measurement curve fitting (linearity of desorption curve) Moisture Modeling 9

10 3D Correction of Diffusivity Estimate for Toroidal O-Ring Moisture Ingress 1D h 3D h 1-D FEA Model Axisymmetric FEA Model Moisture Modeling 10

11 3D Correction of Diffusivity Estimate Moisture / Thermal Analogy A direct analogy can be made between Fick s Law and heat diffusion in order to effectively use FEA software for modeling Correspondence table for thermal/moisture analogy Property Thermal Moisture Primary variable Temperature, T Wetness, w Density ρ (kg/m 3 ) 1 Conductivity k (W/m* C) D*C sat (kg/s*m) Specific heat c (J/kg* C) C sat (kg/m 3 ) *Madenci, Erdogan, and Ibrahim Guven. The Finite Element Method and Applications in Engineering Using ANSYS. Arizona: Springer, Web. < Moisture Modeling 11

12 Analysis Linear Portion of Desorption Curves (25 C) (W(t)-W(0))^ D y = 1.63E-05x y = 5.82E-06x D Time ( x 1000 s) Correction factor for diffusivity coefficient: α correction = m slope(1d) / m slope (3D) =0.36±0.01 Moisture Modeling 12

13 Results 3D D 3D = α correction D 1D =0.36 D 1D Diffusivity (m 2 /s) Materials 25 C 70 C Silicone (5.7 ± 0.5) x (5.3 ± 0.4) x EPDM (4.2 ± 0.9) x (1.36 ± 0.09) x Moisture Modeling 13

Verification of Results FEA Results Fitted to Silicone 70 C 1.00 0.90 0.80 0.70 FEA Results Wetness 0.

14 Verification of Results FEA Results Fitted to Silicone 70 C FEA Results Wetness Measured Time ( x 1000 s) Moisture Modeling 14

15 Conclusions and Future Work Preliminary Conclusions JEDEC recommended geometry is not always feasible for moisture uptake/ desorption specimens. In such cases, modeling is needed to account for 3D moisture flow when extracting diffusion and permeability constants from test data. Different thicknesses and radii for the O-rings do not seem affect the ratio of the 1-D diffusivity versus the 3-D diffusivity using the same thickness At 25 C, EPDM s diffusivity is about two thirds of silicone s diffusivity Silicone s diffusivity increases by a factor of about nine from 25 C to 70 C EPDM s diffusivity increases by a factor of about three from 25 C to 70 C Remaining Work Repeat experiment at different desorption temperatures to obtain activation energy Run further tests and analysis for other materials and geometries Moisture Modeling 15

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